To use a gold testing pen, hold the pen wire under the test metal, rub the point of the pen's tip onto the top of the metal, let the gold metal dissolve into an ink onto the tip of the pen, and use the ink to draw a line on a piece of paper. The color of the ink determines the type and quality of the gold. It's important to only conduct the test on bright metal surfaces.
A resistance to nitric acid identifies most gold and precious metals, but acid can't tell the difference between white platinum gold and other white metals. Many electronic testers also miss white gold and antique yellow gold. Pens work well to identify these types of gold, and can also reveal that some gold has more karats than revealed by acid tests.
When conducting tests, always begin with metals that you know to be gold. Use a cleansing powder with water to remove plated ink from jewelry before testing. To use a pen tester on costume jewelry that might be plated, rub over the same spot five to 10 times. If the jewelry is thinly plated, it shows little to no color after rubbing. Thickly plated gold cracks visibly, and solid gold doesn't crack at all.